Closure Compiler Ignores Object Mutation

Question

I put the following code into closure compiler on advanced mode:

var obj = (function() {
  /** @constructor */
  function H(a) {
    this.a = a
  }
  var h = new H(1);
  h.b=1
  return h
})();

The result I get back is:

(function() {
  var a = new function() {
  }(1);
  a.a = 1;
  return a;
})();

Why is it ignoring the change I make to the object h.b=1 ?

Try adding `this.b = null` in the constructor. Perhaps it's optimized in a way to ignore code that would result in multiple hidden classes? — plalx, Sep 02 '14 at 17:41
@plalx I was hoping to see it add `this.b=1` to the constructor. — qwertymk, Sep 02 '14 at 17:46
While the question by itself is valid, notice that [this pattern shouldn't be used](http://stackoverflow.com/q/10406552/1048572) — Bergi, Sep 02 '14 at 18:41

score 1 · Accepted Answer · answered Sep 02 '14 at 18:25

The advanced compilation options enable aggressive property removal, which includes some assumptions:

It makes a strong assumption that properties defined on a "prototype" or "this" will not be iterated over and thus is a candidate for removal.

/** @constructor */ function cls() { this.x = 1; // removal candidate due to "this" assumption; }

So what you're seeing is actually this.a = a being removed, and then the property b is being renamed to a.

Closure Compiler Ignores Object Mutation

1 Answers1